1. Field of the Invention
The present invention relates to a control apparatus and method, and an engine control unit for an internal combustion engine which is operated with a combustion mode of an air/fuel mixture switched among a plurality of combustion modes.
2. Description of the Prior Art
Conventionally, as a control apparatus for an internal combustion engine, one described in Laid-open Japanese Patent Application No. 10-227239 is known by way of example. This internal combustion engine is of a so-called direct injection type, where a fuel is directly injected into cylinders by fuel injection valves. This control apparatus selectively switches a fuel combustion mode in accordance with a load on the internal combustion engine, i.e., the opening of an accelerator pedal among a first mode for a low load application in which a fuel is injected once in a compression stroke, a second mode for a middle load in which a fuel is injected in each of an intake stroke and a compression stroke in parts, and a third mode for a high load in which a fuel is injected once in an intake stroke. In this way, the internal combustion engine is operated such that an air/fuel mixture is stratified in a low load range, such that part of the air/fuel mixture is stratified while the rest is uniformly burnt in a middle load range, and such that the air/fuel mixture is uniformly burned in a high load range.
Also, this control apparatus executes ignition timing control in the following way. First, one of three ignition timing maps for the first to third modes is selected based on the fuel injection mode. In the ignition timing map for the first mode, a map value is constantly set substantially irrespective of a load, whereas in the ignition timing maps for the second and third modes, a map value is set to a more retarded value as a load is larger. In addition, in two ignition timing maps for load ranges adjacent to each other, map values are set to be discontinuous to each other for a load and to have a relatively large crank angle difference near the boundary of the load regions.
Next, the ignition timing control calculates an ignition timing by searching a selected ignition timing map in accordance with a load. In this event, the ignition timing is calculated through an interpolation of two map search values when the load is in one of the three mode ranges, and when the load is near the boundary of two mode ranges, the interpolation of two map search values is prohibited, and the ignition timing is calculated based only on a single map search value.
The ignition timing is calculated by the foregoing control approach for the following reason. Generally, when a single injection mode such as the first or third mode which involves injecting a fuel only once during one combustion cycle is compared with a split injection mode such as the second mode which involves injecting a fuel twice in parts, the two modes differ in the air/fuel mixture combustion state from each other, as described above, and in thermal efficiency (i.e., combustion efficiency) from each other, thereby causing a large difference in generated torques. As a result, when the fuel injection mode changes between the two modes due to a change in load, this causes a torque step or sudden fluctuations in rotation, possibly leading to a degraded operability. Accordingly, when a load presents a value near the boundary of the two mode ranges, the interpolation of two map search values is prohibited, and the ignition timing is calculated based only on a single map search value to rapidly change the ignition timing, thereby restraining the torque step and sudden fluctuations in rotation to improve the operability.
The control apparatus of Laid-open Japanese Patent Application No. 10-227239 restrains a torque step and sudden fluctuations in rotation when the fuel injection mode changes between two modes by prohibiting the interpolation of two map search values, and employing ignition timing maps which provide map values that are discontinuous to each other for a load and have a relatively large crank angle difference near the boundary of load ranges. However, an increase in torque resulting from an advancing ignition timing is very small as compared with a difference between generated torques in the two modes, and is insufficient for restraining a torque step and sudden fluctuations in rotation. As a result, the operability is still susceptible to degradation due to the torque step and sudden fluctuations in rotation. In addition, since two map values near the boundary must be set to be discontinuous and have a relatively large crank angle difference in order to restrain the torque difference and sudden fluctuations in rotation, one map value must be set to a fairly retarded value, possibly resulting in a lower thermal efficiency and an exacerbated fuel economy.